1. Field of the Invention
The present invention relates to a method for identifying a string with a low-output photovoltaic (PV) module within a larger PV system, and to an associated device for carrying out the method.
2. Description of the Background Art
Large PV systems can be formed of thousands of PV modules, which must be measured individually in order to detect and localize a damaged module. This effort is necessary because the presence of one, or even several, modules that are low-output, if not in fact are faulty, is not directly apparent in the total output. A faulty module in which a photovoltaic cell is non-conductive, or in which the solder joint between two cells is broken, causes failure of the entire string of, for example, ten series-connected PV modules, since a single break also breaks the series connection. For a total photovoltaic system output of 2 MW, the contribution of a single string of, e.g., 2 KW is one one-thousandth of the output. Even several faulty or low-output strings over time are not immediately apparent, since the variation in the output generated can also be weather-related and the measurement accuracy of the current transformers employed is not sufficiently accurate to detect such small variations even in the case of constant weather. Permanently installed systems for output measurement are associated with an unacceptable cost outlay.
In addition to the above problem of undetected low output capacity of the PV system, it is important, especially during the warranty period, for justified claim opportunities to be recognized in order to place responsibility on the manufacturer of the faulty PV module.
In the conventional art, various ways of testing the output capacity of PV modules are known. In all methods that provide a definitive result, the PV system, however, must be disconnected from the inverter and connected to a measuring instrument.
In this regard, multimeters are typically used as the measurement instruments, in that they measure a short-circuit current and open-circuit voltage of a PV module, a string with multiple series-connected PV modules, or a PV unit. The goal of this measurement is to recognize the basic functionality of the PV module, string, or PV unit.
If the output capacity of a PV module, string, or PV unit is to be ascertained, this is preferably done by connecting an instrument for measuring the U-I characteristic, which is capable of measuring the associated U-I characteristic. The characteristic thus measured is supplemented by the measured value of an irradiance sensor or a reference solar cell, as well as the measured value of a temperature sensor that measures the temperature of the PV module. The STC rating (Standard Test Conditions for photovoltaic modules) is calculated from the aforementioned values—irradiance, temperature, voltage, and current. However, this rating has a large inaccuracy as a result of the numerous tolerances of the sensors that are used for the calculation.
Furthermore, measurement with a clamp-on current probe is known from the conventional art, since one can use it to ascertain the current in a PV module, string, or PV unit in ongoing operation. However, since voltage, irradiance, and temperature are unknown, this type of measurement is only sufficient for testing functionality and checking fuses. Thus, the methods and devices from the conventional art produce insufficiently precise results with regard to accuracy, time required, and applicability during operation.